Not applicable.
Not applicable.
The present invention relates to measurement of fluid flow and to such measurements performed by receiving and processing ultrasonic signals that are transmitted into the fluid. It particularly relates to tag flow meters wherein inhomogeneities or turbulence in the fluid itself, or matter such as bubbles, droplets or particles that are moving in the fluid flow, constitute xe2x80x9ctagsxe2x80x9d that modulate the ultrasonic signal. For operation of such tag flow measurement systems, signals are generally launched across the fluid along two parallel paths and received by separate receivers after being modulated by the fluid flow.
This mode of measurement, useful for certain flows that naturally contain, or are seeded, heated, agitated or injected to include, discrete entrained scatterers, offers a number of advantages. By detecting a similar pattern of modulation on the two paths at different times, the fluid velocity may be directly derived from the distance between paths divided by the elapsed time between occurrence of correlated modulation patterns. Thus, baseline calibration steps for amplitude, density, temperature or the like, commonly required for other types of ultrasonic measurements, are not needed. Moreover, tag measurements often are applicable to situations in which the scatterers would introduce too much attenuation or noise for other modes of ultrasonic measurement to be effective. Tag measurements are also useful when most of the interfering noise is stationary, e.g., coherent crosstalk, but not overwhelming.
In general, an effective tag measurement requires that the tags or modulators present in the fluid flow be displaced coherently, i.e., as a group along the direction of fluid flow so that they modulate the signal similarly as they cross each of the interrogation paths. This may require that the paths be located relatively close together with a spacing that decreases with decreasing flow velocity. When the transducers are mounted outside the conduit wall, or are not well isolated acoustically from the conduit wall, as frequently occurs in clamp-on measurement applications, this may result in high levels of noise and crosstalk, substantially degrading the signal to noise ratio and making effective correlation of the two received tag-modulated signals difficult or impossible.
Recently, Chang Shen and Saul Jacobson have proposed, in commonly owned U.S. patent application Ser. No. 09/417,946 filed Oct. 13, 1999, that a clamp-on tag measurement system may be effectively implemented, even for relatively low impedance fluids such as low pressure steam traveling in a steel conduit, by arranging the transmitter to energize a region of the wall and positioning plural receivers to discriminate received signals along different parallel paths of known spacing. That patent application describes a tag measurement method using two different signal frequencies in the two transmitter/receiver pairs. The entire specification of that patent application is hereby incorporated herein by reference. While the approach described therein has extended the feasibility of clamp-on measurements to low pressure gases in noisy conduits, the nature of such clamp-on applications and systems involves a low signal to noise ratio, and in many practical applications involving small pipes or low flow rates, such as in domestic heating plants where a fluid such as wet steam otherwise appears appropriate for tag measurement, the environment itself provides such a level of intrinsic system noise that even the improved transducer and processing configurations described above may fail to yield discernible signals, or fail to provide effective and repeatable correlations.
It is therefore desirable to provide a more effective tag flow measurement system.
It is also desirable to provide an ultrasonic tag flow measurement system applicable to small conduits, and/or noisy environments.
It is further desirable to provide an ultrasonic measurement system adapted to perform either continuous or occassional measurements with certainty and accuracy.
It is also desirable to provide an ultrasonic tag flow measurement system useful for measuring flow of steam, flare gas, and industrial process gases in small conduits.
One or more of the foregoing results are obtained in accordance with the present invention by providing a tag flow measurement system wherein a set of preferably clamp-on transducers define first and a second measurement paths spaced apart along the conduit and directed across a fluid flowing in the conduit such that a receiver in each path receives signals modulated by scatterers in the fluid. The direction of signal propagation in one path is in an opposite sense, e.g., anti-parallel, to the direction of propagation in the other path, and a correlation processor operates on both received signals to determine the flow rate. In one embodiment each path is defined by a transmitter on one side of the conduit and a receiver on the other side of the conduit, with the positions of transmitter and receiver being reversed in the second pair. Thus, typically, the first transmitter lies on the same side of the conduit as the second receiver, and the second transmitter lies on the same side of the conduit as the first receiver. A communicating transmitter and receiver may be diametrically opposed, or may be positioned at opposite ends of a chordal path.
Advantageously, the system may be applied to small conduits having a diameter below 100 millimeters, and it has even been found effective, for example, on a schedule 40 one inch steel pipe carrying low pressure steam or gas. The use of anti-parallel paths greatly reduces coherent crosstalk extending from one transmitter to the receiver of the other pair, and signal to noise ratio may be enhanced by a factor of ten or more, allowing effective correlation of the two received signals. Preferably, each transmitter operates at a different frequency, and the received signals are received and demodulated in phase quadrature to further enhance signal quality. Frequencies or frequency pairs in the range of approximately one to four megahertz may be useful for one inch pipe, while lower frequencies in the range of 0.1 to 0.5 megahertz are suitable for larger conduits.